Conventionally, heater units having a resistance heating element embedded in a metallic base have been widely used in susceptors and electrostatic chucks for semiconductor processing. The resistance heating element typically consists of a sheath heater including a sheath tube made of stainless steel. Typically, a stainless steel sheath heater is cast into an aluminum base, clamped between a pair of aluminum or stainless plates, or interposed between a pair of plates which are joined together by welding along the outer edge.
FIG. 7 shows a conventional cast heater unit 10 including a base 11 having a stainless steel sheath heater 12 cast therein. The stainless steel sheath heater 12 typically comprises a stainless steel sheath pipe, a resistance heater wire received in the sheath pipe, and an insulator which electrically insulates the resistance heater wire from the sheath pipe. The sheath heater 12 is bent into a desired shape, and is cast into the aluminum base 11. The aluminum base 11 is then machined and polished to a desired finish.
FIG. 8 shows a conventional clamped heater unit 13. In this case, the metallic base consists of a lower base 14 and an upper base 15, each made of an aluminum or stainless steel plate. A sheath heater 12 is received in a groove 14a formed in the lower base 14, and the upper base 15 is placed closely over the lower base 14. The upper and lower bases 14 and 15 are then firmly joined with each other by fastening them together with threaded bolts.
FIG. 9 shows a conventional welded heater unit 16. In this case also, the metallic base consists of a lower base 17 and an upper base 18, each made of an aluminum or stainless steel plate. Likewise, a sheath heater 12 is placed in a groove 17a formed in the lower base 17, and the upper base 18 is placed closely over the lower base 17. The upper and lower bases 17 and 18 are then firmly joined with each other by welding them together along their edges 19.
Each of the above described conventional arrangements uses a sheath heater which is bent into a meandering shape, and embedded in the base. It is desirable to sharply bend the sheath heater so that the sheath heater may extend uniformly in a fine pitch to the end of rapidly and uniformly heating the base. However, there is some difficulty in bending the sheath heater at sharp angles without causing excessive stress to the sheath heater.
Also, in the case of the cast heater unit, the metallic base typically consists of aluminum which has a relatively low melting point (typically about 540.degree. C.), and the cast heater unit using such a metallic base is therefore unsuitable for use in a high temperature environment exceeding 500.degree. C. in temperature. Also, because of the sheath tube is typically made of stainless steel which has a substantially different thermal expansion coefficient from that of aluminum, the resulting thermal stress may cause deformation at high temperatures, and repeated applications of such a thermal stress may cause a breaking of the heater wire and/or a deformation of the heater unit.
In the case of the clamped heater unit, a certain gap is created between the upper and lower bases and/or between the sheath heater and the base, and this gap impairs heat transfer. It may prevent a highly responsive temperature control, and may cause localized temperature rises. Localized temperature rises are known to impair the durability of the resistance heater wire.
A welded heater unit is not much different from a clamped heater unit in these respects, and has the additional problem of distortion due to the heat at the time of welding.